CPR feedback systems have recently gained attention as a method for improving the quality of CPR on a cardiac arrest patient. Such systems typically measure the compression depth and rate during chest compressions, compare the measurements with accepted Guideline limits (such as those defined by the American Heart Association in 2005) and give verbal or visual feedback to the rescuer if, for instance, compression depth does not meet the accepted value of 38 mm-50 mm (1.5-2 inches).
A system for giving feedback on compressions typically consists of a sensor pad to be placed on the patient's chest. The sensor pad may contain an accelerometer and optionally a force sensor. The compression depth measurement is usually based on a double integration of acceleration. An additional force measurement can help in signal processing to remove accelerometer offset (see Aase, et. al. 2002), and can also be used to give feedback on incomplete release, as disclosed in EP 1491176 A1.
However, although current Guidelines specify a depth range for compressions, there is no direct relationship between compression depth and CPR efficacy that has been proven to be valid for all individuals.
Human bodies have various forms and sizes, and the same compression depth may not be equally efficient in a large as in a small individual. This uncertainty is qualitatively accounted for in adult CPR Guidelines, where it is acknowledged that a larger individual may require more and a smaller individual may require less compression depth than the recommended value. For children, no definite depth target exist, and the rescuer is advised to compress a certain portion of the chest height of the cardiac arrest patient (see American Heart Association 2005).
However, the acceleration based technology of most CPR feedback systems only measures the stroke length of compressions. The technology is therefore not suitable to assess the size of the patient and compensate compression depth accordingly. In addition, if the patient is placed on a compliant mattress, current systems will detect the entire movement of the chest surface and not only the dimensional change of the chest which is relevant for CPR. Thus, the depth reported by the feedback system will be too large, and the rescuer may falsely believe that she is compressing deep enough.
Many feedback systems give a warning if the rescuer does not release all force from the chest during compressions. This feedback is often based on a measurement of the minimum force alone, or the force in combination with a crude estimation of “chest compliance” to estimate compression depth such as in patent application EP 1491176 A1. However, since the force/depth characteristics for small displacements of the chest vary significantly among individuals, and because the stiffness generally increases progressively with depth, the actual leaning depth associated with such measurements may be quite uncertain.
Accordingly, there is a need for improved chest compression monitoring that accounts for different individual body types and the motion of an underlying surface.